1. Technical Field
The present invention relates to a control method, a fixing device, and an image forming apparatus incorporating the same.
2. Background Art
Fixing devices are employed in electrophotographic image forming apparatuses, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, wherein an image formed of toner particles is fixed in place with on a recording medium such as a sheet of paper.
Among various types of fixing devices known in the art, pressure-assisted thermal fixing devices are widely used. This type of fixing device employs a pair of opposed rotary members, such as cylindrical rollers and endless, looped belts, one being a fuser member subjected to heating, for example, with a halogen heater, and the other being a pressure member pressed against the fuser member to form a fixing nip therebetween, through which the recording medium is conveyed under heat and pressure to fix a toner image thereon.
One problem associated with the fixing device depicted above is undesirable transfer of toner particles from the recording medium due to improper heating of the toner image at the fixing nip, a phenomenon generally referred to as “toner offset”. Toner offset takes place where adhesion between the fuser member and the toner image exceeds adhesion between the recording medium and the toner image, causing a small portion of toner, which should ideally be fixed on the recording medium, to transfer from the recording medium to an adjoining surface of the fixing member.
Two types of toner offset are known: cold offset and hot offset.
Cold offset occurs where the toner image fuses only superficially due to insufficient heating at the fixing nip, leaving an inner portion of the toner layer in a loose, unfused state, which can partially crush up and eventually migrate to the fixing member. Incomplete fixing can also result in image defects as the toner image rubs off readily from the resultant print. Use of toner with a small particle size, which is becoming popular to meet high-quality requirements of modern imaging equipment, can aggravate cold offset, where small toner particles, as opposed to coarse, pulverized toner particles, lodge in microscopic pits on the printed surface of the recording medium, making it difficult to heat and fuse the toner image evenly and sufficiently.
Hot offset, on the other hand, occurs where excessive heating at the fixing nip affects viscoelasticity of the toner image being fused, so that the toner exhibits an excessively high adhesion to the fuser member that exceeds a cohesive force of toner particles, resulting in partial migration of toner to the fuser member.
Not surprisingly, toner offset detracts from image quality not only due to a lack of toner falling off from the recording medium, but also because the resultant print is susceptible to soiling where offset toner, once transferred from the recording medium to the fixing member, is again transferred from the fixing member to another recording medium that enters the fixing nip subsequent to the foregoing recording medium.
Various techniques have been proposed to clean the fixing member of offset toner and other adherent contaminants in the fixing device.
For example, one known fixing device employs a cleaning web, such as an elongated strip of unwoven fabric, to clean a rotary fuser member. The cleaning web is held against the fuser member to wipe offset toner off as the fuser member rotates during operation of the fixing device. For maintaining effective cleaning performance, a web supply mechanism may be provided to supply a new, unused portion of the cleaning web to the fuser member, which prevents escape of offset toner through a minute gap between the cleaning web and the fuser member.
Another, more sophisticated method includes a cleaning system that controls an amount by which the cleaning web is supplied to the fuser member according to image data from which a toner image is produced. The capability to control cleaning web supply allows for reducing maintenance costs and efforts, while obviating an unnecessary, superfluous supply of cleaning web, which would otherwise result in a significant amount of waste material that is detrimental to the environment.
The web-based cleaning system in which the cleaning web is applied to the fuser member is efficient in that it can clean the surface of the fuser member where the offset toner originally builds up. Unfortunately, however, this approach also has drawbacks. One drawback is that direct, sliding contact between the cleaning web and the fuser member causes damage to the surface of the fuser member, causing artifacts, such as vertical streaks, appearing in the resultant print. Another drawback is that the cleaning web cannot remove soiling from the pressure member to which a substantial amount of offset toner may flow from the fuser member, particularly where the fuser member is positioned vertically above the pressure member.
To counteract the problem, one approach is to apply the cleaning web to the pressure member, instead of the fuser member. For example, a fixing device has been proposed in which a cleaning web is position to contact a rotary pressure member from below to clean the pressure member. Cleaning the pressure member in turn maintains the fuser member clean of soiling where offset toner constantly transfers from the fuser member to the pressure member through the fixing nip. Moreover, positioning the cleaning web vertically below the pressure member effectively prevents offset toner from falling off the pressure member to soil the surrounding structure.